Rotor analyzer for an induction motor

ABSTRACT

A rotor analyzer for an induction motor or generator checks and quantifies the integrity of a rotor that is not currently installed within its stator. The analyzer includes an electromagnetic coil that exposes the bars of a rotor to a pulsating magnetic field to induce a current through the bars. At the same time, the rotor is slowly rotated to sequentially expose each bar. A magnetic field created by the induced current in the bars induces an analog signal within a search coil. The analog signal is converted to digital and inputted to a microprocessor system. The system interprets the input data and manipulates it to provide a clear, understandable indication of the rotor&#39;s condition, such as the relative impendence of each bar. The system also determines how many bars are within a rotor having an unknown number of bars.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The subject invention generally pertains to induction motors andgenerators, and more specifically to nondestructive testing of theirrotors.

[0003] 2. Description of Related Art

[0004] Induction motors typically include a rotor that rotates inresponse to a rotating magnetic flux generated by alternating current ina stator associated with the rotor. A rotational speed differential(known as “slip”) between the rotor and the rotating flux induces acurrent through a rotor cage. A rotor cage typically consists of asingle aluminum casting having several conductive bars that run axiallythrough the rotor and are joined at each end by two conductive endrings. Current induced in the bars creates a magnetic flux that opposesthat of the stator, thus providing the rotor with rotational torque.

[0005] Sometimes it is desirable to inspect the integrity of a rotorbefore a new motor is assembled or before considering its use in arebuilt motor. It is especially valuable to know the impedance of eachrotor bar to identify rotor faults such as a cracked bar, separationbetween a bar and an end ring, or porosity of a bar and/or end ring.However, inspecting and identifying such faults is difficult to do, ascast aluminum cages are often cast directly into a laminated steel coreof the rotor.

[0006] To provide a nondestructive test for rotors, an apparatus andmethod disclosed in U.S. Pat. No. 3,875,511 exposes a rotating rotor towhat appears to be a constant magnetic field provided by anelectromagnetic sending coil. The rotor bars crossing the magnetic linesof flux induce a current through the bars. A receiving coil detects theinduced current to provide an analog signal that can be displayed on anoscilloscope.

[0007] An analog display, however, can be difficult to interpret andquantify. For example, in some cases the spacing between adjacent barsis so close that the spikes or peaks of an analog signal may tend to runtogether, thus making it difficult to distinguish one spike or bar fromanother. Similar negative results may occur when the bars are slightlyrecessed below the outer periphery of the laminated core. In such cases,portions of the core overlaying a bar may adversely shield the bar froma sending or receiving coil, and thus reduce the amplitude of the sensedsignal. Also, when the bars are hidden below the outer surface of thecore, a simple analog display may not provide a clear indication of howmany bars are actually in the rotor. Manufacturers of new rotors will,of course, know how many bars are in their own rotors; however, forthose that rebuild motors manufactured by others, the number of bars maybe unknown.

[0008] With an analog display, electrical noise or a stray spike couldbe misinterpreted as another bar. Moreover, with an analog display, itcan be difficult to establish the repeatability of the readings.Repeatability or comparison of one set of readings to a later one can bevaluable not only to establish the credibility of a particular set ofreadings, but also to determine whether a rotor is deteriorating over anextended period of use.

SUMMARY OF THE INVENTION

[0009] To quantify the integrity of a rotor of an induction motor orgenerator, it is an object of the invention to nondestructively create adigital signature that indicates the impedance of each bar of the rotor.

[0010] Another object of the invention is to repeatedly check theimpedance of each bar of a rotor to establish a credible record of therotor's integrity.

[0011] Another object is to create and store a digital record thatindicates the integrity of a rotor, and later reference that record todetermine the extent that the rotor may have deteriorated over anextended period of operation.

[0012] Another object is to determine the number of bars in a rotor bysequentially sensing the impedance of each bar for more than a fullrevolution of the rotor to create a repeating pattern that indicatesthat every bar has been checked at least once.

[0013] A further object of the invention is to create digital raw datathat indicates the impedance of a rotor's bars, and to manipulate thedata by way of a microprocessor to create an enhanced visual indicationof the impedance of each rotor bar.

[0014] A still further object is to induce an electrical current in arotor by varying the current in an electromagnetic sending coil.

[0015] Another object is to sense the current through an electromagneticsending coil to acquire an indication of a rotor bar's impedance.

[0016] Yet another object is to sense the current or voltage ofelectromagnetic receiving coil to acquire an indication of a rotor bar'simpedance.

[0017] Another object is to distinguish one fault from another, whereinone fault is one or more bars having impedance that exceeds apredetermined limit, and another fault is a pattern of gradually varyingimpedance or a number of bars of especially high impedance beingunequally distributed around the rotor.

[0018] These and other objects of the invention are provided by rotoranalyzer that exposes the bars of a rotor to a varying magnetic field toinduce a current through the bars. A digital signal is created thatvaries as a function of the induced current. A microprocessormanipulates the digital signal to provide an enhanced visual indicationof the impedance of each rotor bar.

BRIEF DESCRIPTIONS OF THE DRAWINGS

[0019]FIG. 1 is a perspective cut-away view of a rotor being tested byan exemplary embodiment of a rotor analyzer with some portions of theanalyzer being schematically illustrated.

[0020]FIG. 2 illustrates a plurality of bar signatures, repeatingpatterns, and an example of an enhanced visual indication of theimpedance of each bar of a tested rotor.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0021] Referring to FIG. 1, a rotor 10 is shown in the process of beingnondestructively tested by a rotor analyzer 12. In this example, rotor10 includes an aluminum cage 14 integrally cast within a rotor core 16made of a stack of laminated steel sheets. A rotor shaft 18 is keyed,welded, and/or otherwise fixed to laminated core 16. Rotor cage 14includes several electrically conductive bars 20 that extend between twoopposing end rings 22.

[0022] To check the resistance, impedance, porosity, or othercharacteristic that reflects the integrity of bars 20 and theirconnection with rings 22, analyzer 12 exposes rotor 10 to a varyingmagnetic field 24. This can be accomplished in number of ways, however,in a preferred embodiment a power supply, such as a variac 26 applies analternating voltage (e.g., 20 to 40 volts, 60 Hz) across anelectromagnetic excitation coil 28 to create field 24. As analternative, it may also be possible to create an effective varyingmagnetic field from a pulsating DC voltage or from a moving magnet(oscillating or rotating). Excitation coil 28, in this example, includeswindings 30 of about 300 turns of film-insulated, 19-gage wire wrappedaround a preferably U-shaped, laminated steel core 32.

[0023] Excitation coil 28 is placed near one end of rotor core 16, whilea search coil 34 is positioned near an opposite end. Search coil 34includes windings 36 having about 400 turns of 20-gage wire and isotherwise similar in construction to coil 28. The actual positioning andmounting of coils 28 and 34 can be provided by any of a variety ofconventional brackets or support structures. Regardless of the chosenmounting structure, the ends of the U-shaped core of coils 30 and 36 arepreferably spaced just a few thousandths of inch (e.g., 0.010 inches)away from the surface of rotor 10. This spacing can be an air gap or canbe taken up by some sort of spacer of a non-ferromagnetic material, suchas a plastic bearing pad. Although the actual spacing is not critical,preferably the spacing remains substantially constant throughout thetesting process of analyzer 12.

[0024] In operation, varying magnetic field 24 of excitation coil 28induces a varying electrical current 38 through a first bar 20′. Theother bars 20 and end rings 22 complete the electrical circuit forcurrent 38. Current 38 in bar 20′ creates a secondary magnetic field 40that induces an electrical signal 42 in windings 36 of search coil 34.At the same time, a drive unit 44 (e.g., a set of powered rollers)slowly rotates rotor 10 at about one or two revolutions per minutesrelative to coils 28 and 34. The relative rotation could alternativelybe achieved by having coils 28 and 34 revolve while rotor 10 is heldstationary. Either way allows current 38 to be generally sequentiallyinduced in each bar 20. Although, with closely spaced bars 20 and arelatively wide excitation coil 28, there may be some overlap, wherebysome of current 38 is actually induced in a bar adjacent to bar 20′.Thus, the inducing of current 38 through each of bars 20 is notnecessarily done in sequence or simultaneously.

[0025] An amp meter or a voltmeter 46 (e.g. a Hewlett Packard model xyz)effectively includes an analog to digital converter 48 and samples theanalog voltage (or current) of signal 42 at a predetermined samplingrate. The sampling rate is preferably several times greater than theproduct of the rotational speed of rotor 10 times the number of bars 20.And the product of the rotational speed of rotor 10 times the number ofbars 20 is less than the cyclical frequency of varying magnetic field24. This allows voltmeter 46 to operate at a predetermined sampling ratethat creates several digital signals 50 or values for each bar 20 asrotor 10 rotates, thus providing a plurality of bar signatures (e.g., 52a, 52 b, . . . and 52 n of FIG. 2) for rotor 10. Together, the pluralityof bar signatures 52 a-n comprises a digital signature 54 of rotor 10.Digital signals 50 for bars 20 that create the plurality of barsignatures 52 a-n and digital signature 54 of rotor 10 can be consideredas raw data and, if desired, may be displayed in a table, chart orgraphical format on paper or on a monitor screen 56, as shown in theupper half of FIG. 2.

[0026] Since the raw data can be difficult to interpret, amicroprocessor system 58 (e.g. a computer with the appropriate I/O,microprocessor chip, memory, software, and various other relatedcomponents) receives the raw data or digital signal 50 at an input 60(e.g., a serial port) and manipulates the raw data to provide anenhanced visual indication of the rotor condition, such as the impedanceor other predetermined characteristic of bars 20. An output 62 conveysthe microprocessor-manipulated data to a printer or monitor 56, whichdisplays the information as a bar graph 64, as shown in the lower halfof FIG. 2. Bar graph 64 is just one of many examples of an enhancedvisual indication. Other examples would include, but not be limited to,various other graphical formats; tables; charts; or accept/rejectsignals, such as lights or text.

[0027] To create an enhanced visual indication, such as bar graph 64,microprocessor system 58 first determines the number of bars 20 that arein rotor 10. If the number of bars is already known, the information cansimply be manually inputted to system 58. Otherwise, system 58 analyzesthe raw data it receives at input 60 to identify a repeating pattern ofbar signatures. To create at least a partially repeating pattern, rotor10 is rotated more than one revolution. Although between one and tworevolutions is possible, rotor 10 is preferably rotated three or fourtimes.

[0028] In some embodiments, programmed software of system 58 starts byassuming that rotor 10 has some particular number of bars, say forty.The average pitch or distance 66 between conspicuously clear signalpeaks times forty then identifies what may be a full-cycle or completerotor signature. System 58 then compares that rotor signature to what itconsiders as the next full-cycle of readings. A close correlation of thetwo presumably complete cycles indicates that the rotor actually hasforty bars. The process is repeated for various other reasonable numbersof bars, such as thirty-nine, thirty-eight, forty-one, forty-two, and soon. The closest correlation helps determine the actual number of bars ofan unfamiliar rotor. Of course, system 58 is preferably provided withsome additional logic to eliminate unreasonable numbers of bars. Forexample, it would be very unlikely or unreasonable to suspect that arotor would have just one or two bars. If rotor 10 is rotated less thantwo revolutions, system 58 looks for a correlation between the first fewbar signatures of the first revolution and the first few bar signaturesof the presumed next revolution.

[0029] Once the number of bars 20 has been determined, that same numberin readings will be taken of the raw data at a pitch that most closelyfits a complete rotor signature. And the readings are preferably, butnot necessarily, taken at or near each anticipated peak of each bar 20(i.e., where a peak would normally occur for a good bar). The readingsare then displayed as a first set of discrete digital values to createthe upper portion of bar graph 64 of FIG. 2. For rotor 10 havingtwenty-eight bars 20, a corresponding twenty-eight columns 68 a, 68 b, .. . 68 n are displayed. In other words, bar signature 52 a correspondsto column 68 a, and bar signature 52 n corresponds to bar signature 68n. The higher the column, the lower the impedance of the correspondingbar. If desired, additional readings taken beyond the first revolutionof rotor 10 can also be displayed as a second set of discrete digitalvalues to indicate the repeatability of the readings. The additionalreadings are displayed as columns 68 a′, 68 b′, . . . 68 n′. Here, barsignature 52 a′ corresponds to column 68 a′, and bar signature 52 n′corresponds to bar signature 68 n′. The height similarities of adjacentcolumns, e.g., columns 68 a and 68 a′, provide the indication ofrepeatability. As an alternative, the repeatability of the readingscould also be indicated by a number, such as a ratio of the heights ofcolumns 68 a and 68 a′.

[0030] In a currently preferred embodiment, valleys 90 between each peak92 of the raw data are identified and averaged to provide another set ofdiscrete digital values 92 a-n that are generally lower than the firstset 68 a-n taken near the peaks. This lower set of digital values 92 a-nare shown underneath their corresponding peak values 68 a-n. Comparingtheir relative values, e.g., 68 a/92 a, provides a ratio that can beused as accept/reject criteria for each rotor bar 20. A rotor bar 20 maybe acceptable if its peak-to-valley ratio is above a predeterminedlevel. In some embodiments the predetermined level is relative in thatthe acceptable level is chosen based on how a particular ratio of onebar compares to that of the others. With some defective bars, a distinctvalley may not even exist. For example, 92 n has a value that isvirtually the same as 68 n.

[0031] In some embodiments, rotor analyzer 12 identifies various faultsof rotor 10 based on the microprocessor-manipulated data and/or thesampled raw data. For example, one fault may be defined as a bar havingan impendence that exceeds a predetermined limit. This is graphicallydepicted by the columns associated with bars 20″ being below a minimumconductivity limit 70. If desired, such a fault can be distinguishedfrom other predefined conditions or faults, such as a group of bars ofrelatively poor impendence being unequally distributed about rotor 10.

[0032] In some embodiments, system 58 includes a memory 72 that storesdigital signature 54 and/or 64 for later reference. Memory 72 isschematically illustrated to represent the wide variety of forms that itcan assume, which include, but are not limited to, a hard drive of acomputer; a floppy disc; a CD (compact disk); magnetic tape; and anelectronic chip, such as RAM, EPROM, or EEPROM. With memory 72, adigital signature taken of rotor 10 when first installed within itsstator, can be compared to a later signature taken after rotor 10 hasbeen in operation for a while. The comparison of the two signaturescould indicate whether the integrity of rotor 10 deteriorates with use.On a short-term basis, while inspecting a rotor, memory 72 can be usedin comparing the set of readings taken during the first revolution ofrotor 10 to those of a second revolution, thereby providing anindication of the readings, repeatability.

[0033] Although the invention has been described with reference to acurrently preferred embodiment, it should be appreciated by thoseskilled in the art that other variations are well within the scope ofthe invention. For example, electrical signal 42 is just one example ofa signal that varies as a function of induced current 38. Other examplesof a signal that varies with current 38 include, but are not limited to,amperage 74 or voltage 76 as provided by amp meter 78 and voltmeter 80,respectively. For voltage signal 76, however, variac 26 or another powersupply should be selected so that its output voltage, which it appliesacross excitation coil 30, preferably decreases with an increase incurrent through coil 30. An appropriate analog to digital converter 82converts the analog signal 74 or 76 to a digital signal 50′, which inturn is conveyed to input 60′ or another similar input 60 ofmicroprocessor system 58. System 58 then manipulates signal 50′ in amanner similar to that of signal 50, but with appropriate changes toaccount for any differences between signals 50 and 50′. By using signal74 or 76 instead of signal 42, search coil 34 may be omitted. Inconsideration of such modifications, as well as others that would beobvious to those skilled in the art, the scope of the invention is to bedetermined by reference to the claims, which follow.

We claim:
 1. A rotor analyzer for a rotor having a plurality of bars andbeing normally associated with a stator, comprising: a varying magneticfield independent of said stator and projecting into said rotor toinduce a current through at least one of said plurality of bars and tocreate an electrical signal that varies as a function of said inducedcurrent; an analog to digital converter associated with said electricalsignal and adapted to provide a digital signal that varies as a functionthereof; and a microprocessor system having an input and an output,wherein said input is coupled to receive said digital signal, andwherein said output provides microprocessor-manipulated data that variesas a function of said digital signal to provide an enhanced visualindication of a certain characteristic of said plurality of bars.
 2. Therotor analyzer of claim 1, further comprising an excitation coil adaptedto conduct a pulsating current to create said varying magnetic field. 3.The rotor analyzer of claim 2, wherein said electrical signal is saidpulsating current.
 4. The rotor analyzer of claim 1, further comprisinga search coil exposed to a secondary magnetic field created by saidinduced current.
 5. The rotor analyzer of claim 4, wherein saidelectrical signal is induced within said search coil.
 6. The rotoranalyzer of claim 1, wherein said enhanced visual indication includes afirst plurality of discrete digital values in one-to-one correspondencewith said plurality of bars, thereby establishing a digital signature ofsaid rotor.
 7. The rotor analyzer of claim 6, further comprising amemory adapted to store said digital signature for later reference. 8.The rotor analyzer of claim 6, wherein said enhanced visual indicationincludes a second plurality of discrete digital values in one-to-onecorrespondence with said plurality of bars, whereby said first pluralityof discrete digital values and said second plurality of discrete digitalvalues provides an indication of repeatability of said digital signal.9. The rotor analyzer of claim 1, wherein said certain characteristicincludes bar impedance.
 10. The rotor analyzer of claim 1, wherein saidenhanced visual indication allows one fault to be distinguished fromanother fault, wherein said one fault is at least one of said pluralityof bars having a bar impedance that exceeds a predetermined limit. 11.The rotor analyzer of claim 1, wherein said analog to digital converterprovides a plurality of digital signals for each of said plurality ofbars to create a bar signature for each of said plurality of bars,thereby creating a plurality of bar signatures.
 12. The rotor analyzerof claim 11, wherein said analog to digital converter provides aplurality of bar signatures in excess of said plurality of bars tocreate at least a partially repeating pattern of said bar signatures,said microprocessor system determining the number of said plurality ofbars of said rotor based upon said at least a partially repeatingpattern.
 13. The rotor analyzer of claim 1, further comprising a driveunit adapted to engage said rotor and adapted to rotate said rotor at arotational frequency, wherein a product of said rotational frequencytimes the number of bars of said plurality of bars is less than acyclical frequency of said varying magnetic field, thereby creating aplurality of digital signals for each of said plurality of bars tocreate a bar signature for each of said plurality of bars.
 14. A methodof analyzing a rotor having a plurality of bars, comprising: inducing acurrent through each of said plurality of bars, but not necessarily insequence or simultaneously; creating raw digital data as a function ofsaid current; manipulating said raw digital data to provide an enhancedvisual indication of a certain characteristic of said plurality of bars.15. The method of claim 14, wherein said enhanced visual indicationincludes an individual digital value assigned to each of said pluralityof bars to provide a digital signature of said rotor. 16 The method ofclaim 15, wherein said enhanced visual indication includes a secondindividual digital value assigned to each of said plurality of bars andfurther comprising comparing said individual digital value to saidsecond individual digital value to provide a peak-to-valley ratio. 17.The method of claim 15, providing a plurality of digital signals foreach of said plurality of bars to create a bar signature for each ofsaid plurality of bars, thereby creating a plurality of bar signatures.18. The method of claim 17, further comprising determining the number ofsaid plurality of bars by recognizing a repeating pattern of saidplurality of bar signatures.
 19. A method of analyzing a rotor having aplurality of bars, comprising: using an excitation coil to induce apulsating current sequentially through each of said plurality of bars;exposing a search coil to a magnetic flux generated by said pulsatingcurrent to create an analog signal; creating raw digital data from saidanalog signal; determining a number of said plurality of bars byrecognizing a repeating pattern associated with said raw digital data;as a function of said raw digital data, assigning a first digital valueto each bar of said plurality of bars; and displaying said first digitalvalue to provide a visual indication of a certain characteristic of saidplurality of bars.
 20. The method of claim 19, further comprisingassigning a second digital value to each bar of said plurality of bars,wherein said second digital value is a function of said raw digitaldata, whereby said first digital value and said second digital valueprovide an indication of repeatability of said analog signal.
 21. Themethod of claim 19, further comprising distinguishing one fault fromanother fault, wherein said one fault is at least one of said pluralityof bars having a bar impedance that exceeds a predetermined limit.
 22. Amethod of analyzing a rotor having a plurality of bars, comprising:establishing a first plurality of values corresponding to said pluralityof bars; and creating a bar chart having a first plurality of columnscorresponding to said first plurality of values, whereby the associationof said first plurality of bars and said bar chart is readily apparent.23. The method of claim 22, further comprising printing said bar chart.24. The method of claim 22, further comprising displaying said bar charton a monitor.
 25. The method of claim 22, further comprising:establishing a second plurality values corresponding to said pluralityof bars; creating a second plurality of columns corresponding to saidsecond plurality of values; and comparing said first plurality of valuesto said second plurality of values.
 26. The method of claim 25, whereinsaid comparing provides an indication of repeatability of said firstplurality of values and said second plurality of values.
 27. The methodof claim 25, wherein said comparing helps provide an acceptancecriterion for said plurality of bars.